My active whip used a U-310, not a J-309. Intercepts of my active whip
in the MW band were typically +43 dBm OIP3 and +83 dBm OIP2. They
might be slightly lower at the frequencies Steve used, but unlikely to
be 10 dB lower. Perhaps the J-309 gives lower intercepts than a U-310,
or circuit or measurement mistakes were made. Dallas
That's not at all unlikely. It would be very useful to do a bench
comparison to see how various devices compare. I've gone to great lengths
in the past to select bipolar transistors for linearity by testing a few of
each on a curve tracer to see how they compare, and many years ago wrote an
extensive article in QEX on the linearity of bipolars. I should perhaps due
a similar study of JFETs, especially since the J309/J310 datasheet from
Fairchild only shows one curve family and it's supposedly applicable to both
devices.
But, I'm not entirely fond of FETs, mostly due to the wide range of
cutoff voltages which makes them more difficult to deal with than bipolars
when it comes to biasing and coupling them directly to other devices, and
especially when using discretes as differential pairs.
Although I believe that I understand the mechanism that is the root
cause of the rise in IMD when feedback is used for linearization, I'm not
entirely convinced that a source/emitter follower pair is a good overall
solution for an active antenna as it lacks gain, despite the fact that it
provides a high IP3 and IP2. It does provide a suitable interface between
the antenna and the cable or subsequent amplification, but it does so as the
expense of signal loss.
I'm now looking at using dual-gate MOSFETs as they are inherently far
more linear than JFETs. There are some interesting circuitry configurations
derived from pentode vacuum tubes that can be applied to both source
followers and balanced amplifiers that improve the linearity even further.
Chris
,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo\ Sonoran Radio Research
(__)\ _ P.O. Box 25240
\ \ .' `. Tempe, Arizona 85285-5240
\ \ / \
\ '" \ IEEE Senior Member #40274515
. ( ) \
'-| )__| :. \ Email: christrask@...
| | | | \ '.
c__; c__; '-..'>.__
Graphics by Loek Frederiks
